This invention relates generally to binder compositions which are useful in bonding materials such as aggregates to form shaped articles and are particularly useful in bonding foundry aggregate such as sand in making foundry cores and molds. The binder compositions to which the invention relates comprise a phenolic resin component, a polyisocyanate hardener component and a curing agent and are capable of being cured at room temperature. This type of binder composition is well-known in the prior art: U.S. Pat. No. 3,409,579 and U.S. Pat. No. 3,676,392 are examples of detailed disclosures of these binder compositions; the disclosures of these patents are hereby incorporated herein by reference. Generally, the resin component comprises an organic solvent soluble phenolic resin which is the condensation product of a phenol having the general formula ##STR1## wherein A, B, and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals, or halogen, with an aldehyde having the general formula R'CHO wherein R' is a hydrogen or a hydrocarbon radical of one to eight carbon atoms. The phenolic resins employed in these binder compositions can be either resole or A-stage resins or novolac resins. The resitol or B-stage resins, which are a more highly polymerized form of resole resins, have generally been unsuitable because they are solid resins. The phenolic resins useful in these binder compositions have been somewhat further limited in that they must be liquid or must be organic solvent soluble. Although both the resole resins and the novolac resins which meet the above criteria can be employed in these binder compositions, the novolac resins have generally been preferred over the resole resins, because many resole resins are difficultly soluble in the desirable organic solvents and many have a higher water content than has been generally desired in the past for these binder compositions. The preferred novolac resins have been those in which the phenol is prevailingly polymerized through the two ortho positions.
Benzylic ether resins, which comprise one species of the phenolic resins preferred in these binder compositions, are characterized by containing a unit having the formula ##STR2## wherein A, B, and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals or halogen, the R's are individually H or hydrocarbon radical of one to eight carbon atoms, and the average degree of polymerization, as measured by the number of repeating aromatic rings, is generally 3 to 100 and preferably 4 to 10. Although higher molecular weight resins are operable in these binder compositions, such resins are difficult to handle from the standpoint of viscosity in requiring excessive amounts of solvents to bring the viscosity of the resin component to a level normally desired. Due to sand flowability, the speed of cure, strengths, and shake-out characteristics, the especially preferred phenolic resins for foundry binders have been the benzylic ether resins disclosed and described in detail in U.S. Pat. No. 3,485,797, and having the following general formula: ##STR3## wherein R is hydrogen or a phenolic substituent as described in the above-mentioned patents, the sum of m and n is at least two, the ratio of m to n is at least one, and X is an end group selected from the group consisting of hydrogen and methylol, with the molar ratio of said methylol to hydrogen end groups being at least one.
The hardener component of these binder compositions is fully described in above-mentioned '579 and '392 patents and comprises an aliphatic, cycloaliphatic, or aromatic polyisocyanate having preferably from two to five isocyanate groups. If desired, mixtures of polyisocyanates can be employed. Isocyanate prepolymers formed by reacting excess polyisocyanate with a polyhydric alcohol, e.g., a prepolymer of toluene diisocyanate and ethylene glycol, can be employed. Suitable polyisocyanates includes the aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4'-dicyclohexylmethane diisocyanate, and aromatic polyisocyanates such as 2,4- and 2,6-toluene diisocyanate, diphenylmethyl diisocyanate, and the dimethyl derivatives thereof. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylylene diisocyanate, and the methyl derivatives thereof, polymethylene polyphenyl isocyanates, chlorophenylene-2,4-diisocyanate, and the like. Although all polyisocyanates react with the phenolic resin to form a cross-linked polymer structure, the preferred polyisocyanates are aromatic polyisocyanates and particularly the diphenylmethane diisocyanates, the trimethylene triphenyl triisocyanates, and mixtures thereof.
The curing agents useful in these binder compositions include tertiary amines as described in U.S. Pat. No. 3,409,579, the base catalysts described in U.S. Pat. No. 3,676,392, and metal ion catalysts described in U.S. Pat. No. 3,429,848. The catalyst and the amount thereof to be used in these binder compositions is selected by one skilled in the art, depending upon the use of the binder composition, the work-time desired, and the curing time desired. For example, when using these binder compositions in a foundry application a tertiary amine vapor will be used as described in the '579 patent when long work times are desired and essentially instanteous curing times are desired, such as in a typical foundry cold box operation; the base catalyst of the '392 patent or the metal ion catalyst of the '848 patent would be employed when a controlled ratio of work time to strip time is desired, such as in a foundry no-bake operation.
It is taught throughout the prior art relating to these binder compositions that the presence of water is detrimental to the performance of the binder. It is generally taught that the phenolic resin used in these binder compositions should be less than 5% by weight of water and preferably less than 1% by weight of water. In addition, it is generally taught in the prior art that the water content of the material being bonded with these binder compositions should be very low. For example, for foundry sand the moisture content should be less than 1% by weight based upon the weight of the sand. The reason for the necessity of low water levels in the binder composition and the material being bonded with the binder has been that the presence of water interferes with the isocyanate and with the urethane-type reaction involved, thereby weakening the effectiveness and performance of the binder composition.
It is generally taught in the above-mentioned prior art that the binder compositions to which this invention relates must be used in organic solvents. The solvent is necessary in order to evenly distribute the active components of the binder composition on the material or aggregate being bonded with the binder composition. These solvents are generally disclosed in the above-mentioned '579, '392 and '848 patents. Other specific solvent systems for these binder compositions are disclosed in U.S. Pat. No. 3,726,867 and U.S. Pat. No. 3,905,934. While these organic solvents of the prior art provide these binder compositions with excellent performance characteristics, these solvents have the disadvantage of being atmosphere polluting. For example, the solvents can evaporate and escape into the atmosphere during the mixing of the binder compositions with the material or aggregate to be bonded. In a foundry operation the organic solvents contribute to the emissions which occur during the metal pouring.